The present invention relates to a filtered venting system for a reactor containment vessel of a nuclear power plant.
A boiling water type nuclear power plant is well known as one type of conventional nuclear power plant, and a typical example of the boiling water type nuclear power plant, hereinafter called BWR, is shown in FIG. 2. Referring to FIG. 2, a stand-by gas treatment system (SGTS) is installed in a reactor building 1, and this SGTS 2 operates, when a loss of coolant accident (LOCA) as design basis accident (DBA) occurs, to maintain a pressure inside the reactor building 1 to be negative by releasing an ambient atmosphere in the reactor building 1 into an external atmosphere gradually by a small amount and to thereby enclose radioactive substance leaking from the reactor containment vessel (RCV) within the reactor building 1, thus preventing the radioactive substance from being released into the environmental atmosphere.
Since the SGTS 2 is an equipment for a counter-measure to the LOCA as the DBA, the SGTS 2 is considered to be an engineered safety features, and accordingly, it is required for the SGTS 2 to apply a single accident standard or basis to dynamic equipment or mechanisms and to have an anti-earthquake design, which results in the requirement of a highly reliable design thereof.
The SGTS 2 for releasing the ambient atmosphere in the reactor building 1 is provided with an expensive filter train 4 with high performance for removing the radioactive substance in the ambient atmosphere. The filter train 4 includes in a row a pre-filter, a high efficiency particulate filter and a charcoal filter. The filter train 4 acts to sufficiently remove the radioactive substance from the exhausted atmosphere in the reactor building 1 fed by the operation of outlet or exhaust fans 5 and thereafter to discharge cleaned atmosphere including no radioactive substance into the external atmosphere through a stack 6. In the SGTS 2, an emergency a.c. power source such as emergency diesel generator (DG) is utilized for driving electrically operative valve means 7 for the outlet fans 5. In FIG. 2 reference numeral 8 denotes a moisture separator or remover, reference numeral 9 denotes a heating coil and reference character AO denotes an air operating valve.
Recently, in nuclear power plants installed in European countries, a filtered venting system 10 such as shown in FIG. 3 is provided in the reactor building 1 in consideration of an occurrence of a severe accident over the DBA.
The severe accident is an accident which occurs in an assumption of an extremely severe state such as in which the function of an external electric source is lost and all of the plurality of emergency a.c. power sources (DG) are also lost, and concretely, all of the a.c. power sources become entirely unusable, called an accident of loss of all a.c. power source. At the time of such a severe accident, before the inner pressure of the RCV 3 has increased extremely by decay heat generated by a nuclear fuel, a rapture disk 11 of the filtered venting system 10 is automatically operated to thereby discharge the atmosphere in the RCV 3 into the external atmosphere through the stack, thus preventing the RCV 3 from being damaged by the increased pressure.
The radioactive substance in the RCV 3 is removed by a filter vessel 12 equipped in the filtered venting system 10. The filter vessel 12 as a filtering device contains a water filter and a stainless filter, through which the radioactive substance contained in the inner atmosphere is fully removed and the atmosphere including substantially no radioactive substance is then discharged into the external atmosphere.
However, such a filtered venting system 10 has been designed to countermeasure a severe accident, which is a rare accident beyond the DBA, and therefore, it will be said that the design has not been made by specifically paying attention to the application of a single accident standard to a general dynamic equipment and to the assurance of the reliability of the system such as anti-earthquake design. Nevertheless, it is assumed that the accident of loss of all power source resulting in the severe accident is mainly caused with high possibility due to the earthquake, and in the prior art any redundant design or suitable countermeasure is not made in this view point. Accordingly, the described prior art system is not satisfactory in the reliability in an assumption of a case of a severe accident.
Furthermore, the filtered venting system for the severe accident such as shown in FIG. 3 is not designed to be automatically operated at the time of an occurrence of the severe accident. Thus, during steady operation of the reactor, it is necessary for an operator to open isolation valves 13, now being closed, in accordance with a personal judgement of the operator, and this involves much load to the operator. In this point of view, too, it will be said that the system shown in FIG. 3 is also not designed with high reliability. In FIG. 3, reference character L denotes a level meter.
On the contrary, as described hereinbefore, to the SGTS 2 for the design standard accidents designed for the DBA, the redundant design and the anti-earthquake design have been made to thereby automatically operate at an occurrence of the LOCA. In this view point, the SGTS may be said to have a design ensuring the sufficient reliability. However, the SGTS 2 requires the location of the a.c. power source for operating dynamic equipments such as outlet fans 5 and the electrically actuating valves 7, and accordingly, such system 2 cannot be operated at all at the severe accident caused by the accident of loss of all a.c. power source. For this reason, in spite of the provision of the expensive SGTS 2 for the DBA, in the European countries, an independent filtered venting system such as system 10 is additionally provided for the specific countermeasure to the severe accident, resulting in additional working and involving much cost. Moreover, since the filtered venting system is a system for the specific severe accidents, when it is intended to incorporate such filtered venting system 10 in the existing nuclear power plant, there is a fear of giving an erroneous understanding to the public such that the existing power plant is a type liable to cause such severe accident.
In the actual technology, the SGTS 2 is only used for the DBA and the filtered venting system 2 is only used for the severe accident, so that the additional application of the filtered venting system 10 to the SGTS 2 not only requires much additional cost but also gives the wrong understanding to the people.
Furthermore, since the filtered venting system 10 is a system for the specific severe accident and the reliability thereof is less than that of the SGTS 2, in actual, the introduction of the filtered venting system 10 applies much load to the operator, thus providing a problem.